1. Field of the Invention
The present invention relates generally to a defibrillator and, more particularly, to defibrillators that provide variable waveforms.
2. Related Art
An external defibrillator is a device used to administer a high intensity electrical shock through two or more electrodes, commonly referred to as xe2x80x9cpaddlesxe2x80x9d or xe2x80x9cpads,xe2x80x9d to the chest of a patient in cardiac arrest. Energy typically is stored in a charge-storage device (e.g., a capacitor) and is then electrically discharged into the patient through the electrode circuit.
If an initial attempt at defibrillation is not successful, one or more additional attempts typically are made. However, repeated defibrillation attempts, particularly if they are made at increasing levels of intensity, are increasingly likely to cause damage to the heart or other body tissue. Although the threshold levels for damage are not well quantified, it appears that there is not a great deal of margin between an effective defibrillation level and a damaging defibrillation level. Also, the delay associated with repeating the defibrillation procedure may allow the patient""s condition to deteriorate. For example, metabolic imbalance and hypoxia may develop in response to prior attempted resuscitations. Moreover, the development of these conditions typically makes it more difficult to defibrillate the patient and, even if defibrillation is achieved, reduces the prospect of successful recovery. Thus, early and optimal selection of various waveform parameters is crucial to improving the chances of a successful outcome.
One set of waveform parameters thought to be important in determining the safety and success of the defibrillation procedure are those that define the shape of the defibrillation waveform. Waveforms having a variety of shapes have conventionally been used. Some defibrillators employ monophasic (single polarity) voltage pulses. Others employ biphasic (both positive and negative polarity) pulses. Monophasic or biphasic pulses may be damped-sinusoidal, truncated-exponential, constant xe2x80x9ctiltxe2x80x9d (a measure of the difference between the start and end voltage, often expressed as the difference between the initial and final voltages, divided by the initial voltage), combinations of such forms, and so on. Many other forms, such as rectilinear pulses, are possible. In addition, the shape of a waveform may be adjusted by varying its amplitude or duration, or the amplitudes or durations of one or more of its constituent parts. Some conventional approaches for determining what are considered to be optimal shapes for defibrillation waveforms, delivered by both implanted and external defibrillators, are described in U.S. Pat. No. 5,431,686 to Kroll et al., U.S. Pat. No. 4,953,551 to Mehra et al., and U.S. Pat. No. 4,800,883 to Winstrom.
The choice of waveform shape also may depend on whether the defibrillator is implanted or is external. If the defibrillator is implanted, the patient""s unique electrical characteristics and overall physiology may be investigated and the waveform tailored to that particular patient""s needs. External defibrillators, in contrast, are intended to be applied to numbers of patients that have generally varying physiological characteristics. Moreover, a patient may require different waveforms for optimal operation depending, for example, on the contact that is achieved between the electrode and the patient. Thus, external defibrillators may be designed for optimal use on an average patient. Alternatively, they may be designed so that they are capable of providing a variety of waveforms depending on an evaluation of the patient""s physiology, the electrical connection achieved between the electrode and the patient, new knowledge about the operation and affect of electrotherapeutic discharges, or other factors.
Several factors have been used to determine the defibrillation waveform parameters. In particular, many defibrillators presently in use are designed to deliver one or more specific quantities of energy, typically measured in joules, to the patient""s heart. With respect to external defibrillators, practical considerations have contributed to an emphasis on energy-based defibrillation methods. In particular, energy is a relatively easy quantity to control at the power levels and pulse width""s required for transthoracic defibrillation.
Guidelines of the American Heart Association applicable to external defibrillation suggest that a first discharge be administered to deliver a total energy of 200 joules to the patient, a second discharge be administered to deliver 200 to 300 joules, and a third discharge be administered to deliver 360 joules. In conformance with these guidelines, many conventional external defibrillators are designed to deliver these quantities of energy to a patient assuming a typical transthoracic impedance (e.g., 50 ohms). Other defibrillators take into account the variability of transthoracic impedance from one patient to another. In general, these defibrillators measure the transthoracic impedance of the patient and adjust the amount of energy stored in a discharge capacitor or other energy storage device in order to achieve a desired amount of energy applied to the patient""s heart. Some of these conventional defibrillators also vary the shape of the defibrillation waveform as a function of transthoracic impedance and the quantity of energy to be delivered. The rationales for these and other conventional energy-based approaches are described in numerous sources such as U.S. Pat. No. 4,771,781 to Lerman, U.S. Pat. No. 5,620,470 to Gliner, et al., U.S. Pat. No. 5,607,454 to Cameron, et al., and International Application PCT/US98/07669 (PCT International Publication No. WO 98/47563).
The Lerman patent also describes another type of conventional design in which the defibrillation discharge is determined based on current delivered to the patient. In particular, Lerman describes a method for calculating a level of energy necessary to deliver to the patient an amount of peak current pre-selected by an operator. A measured transthoracic resistance of the patient, together with the selected peak defibrillation current, are used to control the charge that is applied to a discharge capacitor of the defibrillator. Upon discharge, the selected level of peak current is applied to the patient. U.S. Pat. No. 4,840,177 to Charbonnier, et al., also describes a method for determining a charge level for an energy storage device such that, when the device is discharged, a desired current flows into the patient. These and other conventional current-based designs seek, among other things, to limit or avoid the damage that may be inflicted by the delivery of an excessive amount of energy. For example, in situations in which the transthoracic resistance is low, a particular selection of energy for discharge into the patient will result in a larger applied current than would be realized if the transthoracic resistance had been high. On the theory that it is the application of current, rather than energy per se, that achieves the desired defibrillation, the energy discharged into a low-resistance patient therefore may be selected to be less than it would be for a high-resistance patient. Thus, the supposed therapeutic benefit is achieved while exposing the patient to a level of energy that is thought to be less likely to cause damage. Various other conventional techniques for determining defibrillation discharge parameters based on operational parameters such as desired energy, current, and/or shape are noted and discussed in the above noted PCT Publication No. 98/47563.
Although current-based defibrillators are feasible, they typically must operate over a wide range of energy and power in order to deliver a specified current over a wide range of possible transthoracic impedances. These requirements often complicate the design of conventional current-based defibrillators. Moreover, it is not clear that the delivery of current, per se, is the mechanism that achieves defibrillation. (See Charbonnier, xe2x80x9cExternal Defibrillators and Emergency External Pacemakers,xe2x80x9d Proceedings of the IEEE, vol. 84, number 3, pages 487-499, particularly at pages 491-93.) Up to a certain point, a longer current pulse requires less peak current to be effective. Thus, the inventor has concluded that defibrillation may be achieved as a result of the accumulation of charge (current over time) rather than by the current per se. Further support for this view may be deduced from what is known of the defibrillation mechanism at the cellular level. The cell walls of heart muscle tissue, like other cells in the human body, have a capacitance. Defibrillation is thought to be accomplished by cell depolarization and introduction of a refractory period. (See Jones, et al., xe2x80x9cCellular Excitation with High-Frequency Chopped Defibrillator Waveforms,xe2x80x9d Proceedings of the 16th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, (IEEE, 1994), pages 17-18.). The inventor concludes that defibrillation may be accomplished by producing a voltage potential difference across the cell wall, and that this voltage potential difference depends on the amount of charge provided rather than on the current or energy levels applied, per se.
Accordingly, in one aspect of the present invention, a method for delivering a desired quantity of electric charge to a patient is disclosed. The term xe2x80x9cdesiredxe2x80x9d in this context means that it is an objective of the apparatus or method of the invention to deliver to the patient a particular quantity, or dosage, of electric charge.
In some embodiments of the method, the desired charge is predetermined. The term xe2x80x9cpredeterminedxe2x80x9d in this context means that, in some implementations of the present invention, the operator of the defibrillator does not select a desired charge. Rather, a default value of electric charge is assumed to be the desired value. As described below in accordance with an illustrated embodiment, this default value may be stored in a memory location accessible by a microprocessor that determines the duration, amplitude, form, and other waveform parameters such that the desired value of electric charge is delivered to the patient""s heart. The default value may also be stored in firmware or determined by configurations and/or values of hardware components.
In some cases, the operator may desire to deliver a quantity of charge different than a predetermined, or default, value. In these cases, the desired quantity of electric charge is referred to herein as being xe2x80x9coperator-selected.xe2x80x9d Some reasons that the operator may desire to select a quantity of electric charge include that application of a default value has not achieved the desired therapeutic effect, that new research or experience indicates that the default value is no longer the best choice in general, that new research or experience indicates that the default value is no longer the best choice in view of certain characteristics of the patient (e.g., weight), and so on. It is not precluded that a default value may be changed due, for example, to new research or experience. In such cases, the default value may be reprogrammed in accordance with known techniques such as by changing values in software or firmware, or by changing hardware components.
The method includes the step of determining intended waveform parameters based at least in part on a desired quantity of electric charge. The intended waveform parameters may also be based at least in part on one or more patient impedances. Waveform parameters may include the form, duration, or amplitude of a waveform. As described in greater detail below, these waveform parameters may be determined in various ways. The word xe2x80x9cdeterminedxe2x80x9d in this context may mean that the parameters are calculated (such as, for example, computing the necessary amplitude and/or duration of a rectilinear voltage pulse such that a desired quantity of current over time, i.e., charge, will be delivered to a patient of a certain impedance). Also, xe2x80x9cdeterminedxe2x80x9d may refer to the application of any of a variety of other known techniques that may be employed to select, retrieve, or in any other way identify waveform parameters that would provide the desired charge if a waveform having these parameters were applied to a patient with a certain impedance. Some examples of other techniques, described in greater detail below, include using a look-up table, or search and compare techniques, to find templates of appropriate model waveforms as stored, for example, in computer memory.
The method also includes the step of generating an applied defibrillation waveform based on the intended waveform parameters. That is, a waveform is generated for application to the patient in accordance with the intended waveform parameters. However, in some implementations, these two functions could be combined into a single function in which the determining and generating of a defibrillation waveform are combined. For example, an operator could employ an electromechanical switch that selects one of two charged capacitors (or selects one of two voltages to which a single capacitor is charged) and discharges the selected capacitor (or selected voltage) into the patient. In this simplified example, the xe2x80x9cdeterminingxe2x80x9d of the intended waveform is accomplished by selecting the capacitor (which may be predetermined to provide one of two desired quantities of charge into a patient of an assumed impedance) and the xe2x80x9cgeneratingxe2x80x9d of the waveform is accomplished by enabling the selected capacitor to discharge into the patient. In an even simpler example, one capacitor or voltage could be used based on a predetermined (e.g., pre-calculated) voltage that will provide a desired quantity of charge to a patient of an assumed impedance.
In some implementations, a further step in the method is that of electrically coupling the applied defibrillation waveform to the patient. This step typically may be accomplished by an operator applying the electrode to the patient. Also, when the electrodes have already been applied to the patient, this step may be accomplished when the operator activates an activator that, among other things, closes a patient isolation relay so that an electrical circuit from the defibrillator to the patient is completed.
A patient may be assumed to have, or may be determined to have, more than one impedance value. This situation may occur for several reasons. For example, a patient impedance value may be assumed or estimated in advance (i.e., predetermined) based, for instance, on average patient impedance values. Also, an operator may select a patient impedance value from one of two or more predetermined values. In the case of an external defibrillator, the estimation or selection of these values may reflect various assumptions regarding a typical value of transthoracic impedance. For instance, the value may be selected to be 50 ohms, 80 ohms, or another value that may be thought to more accurately represent the physiology of a patient population. Another reason that there may be more than one patient impedance value is that, in the case of an external defibrillator, the electrical characteristics of the connection between the electrode and the patient may change during the time that the defibrillation waveform is applied, or from one application to another. This change may result, for example, from variations in pressure or placement of the paddles. Also, the patient""s physiology may be altered by the application of the defibrillation discharge or for other reasons.
Yet another reason for variations in patient impedance value is the difference circumstances applicable to external and implanted defibrillators. As is evident, a transthoracic patient impedance value will be different than a patient impedance value presented to an implanted defibrillator in which the defibrillation waveform is applied directly to the heart. Thus, in implementations of the method involving external defibrillation, the patient impedance is a transthoracic impedance and, in implementations involving internal defibrillation, the patient impedance is a heart impedance.
In addition, variations in patient impedance values may result from measurements of a particular patient""s impedance at one or more times. Thus, in some aspects, the invention is a method that also includes the step of determining at least one of the patient impedance values. This determination may be made in various ways. In accordance with one technique, a value is sensed that is indicative of an impedance of the patient prior to the electrical coupling of the applied defibrillation waveform to the patient. In accordance with another technique, a value is sensed that is indicative of an impedance of the patient substantially contemporaneous with starting the electrical coupling of the applied defibrillation waveform to the patient. This technique also includes determining an adjustment, if any, to the intended waveform parameters based on the sensed value. The purpose of this adjustment is to apply the desired quantity of charge to the patient. Thus, the applied defibrillator waveform is adjusted based on this determination.
The method may also include the steps of sensing one or more values indicative of one or more impedances of the patient during electrical coupling of the applied defibrillation waveform to the patient and determining an adjustment, if any, to the intended waveform parameters based at least in part on the sensed one or more values. The applied defibrillation waveform is adjusted based on this determination. There may be a number of these adjustments made during the application of the defibrillation waveform. For example, it may be sensed shortly after initiation of the defibrillation discharge into the patient that the patient""s impedance has changed from an initial sensed value. The applied defibrillation waveform is adjusted accordingly. Subsequently, during the same defibrillation discharge, it may be sensed that the patient""s impedance has again changed, and thus the applied defibrillation waveform is again adjusted. As noted, these adjustments are made so that the desired quantity of charge is applied to the patient notwithstanding the changes in the patient""s impedance.
In some aspects, the invention is a method that includes comparing the intended waveform parameters with the applied waveform parameters of the applied defibrillation waveform during electrical coupling of the applied defibrillation waveform to the patient. When a difference between the intended and actual waveform parameters reaches a threshold value, the method includes adjusting the applied waveform parameters of the applied defibrillator waveform to conform with the intended waveform parameters. Also, in some aspects, the determination of intended waveform parameters may include determination of any of the following parameters: form, phase, timing of phase transition, maximum duration, minimum duration, maximum voltage, minimum voltage, maximum current, minimum current, maximum energy, minimum energy, maximum power, and minimum power. It will be understood that these intended waveform parameters are illustrative only, and that any other parameter for describing, specifying, modeling, or otherwise representing a waveform may be employed as a waveform parameter in accordance with the invention.
The applied defibrillation waveform in various aspects of the invention includes a set of voltage values. This waveform may include, for example, a monophasic voltage pulse, a biphasic voltage pulse, etc. The applied defibrillation waveform in various aspects of the invention may also include a set of current values.
In other aspects, the invention includes a method for delivering a desired quantity of electric charge to a patient. The desired quantity of electric charge may be predetermined, or it may be operator-selected. This method includes the steps of providing a flow of current over time through an electrical coupling to the patient, and stopping the current flow when a desired quantity of electric charge has been delivered. In some implementations, the step of providing a flow of current over time includes determining intended waveform parameters of the current waveform. This determination may be based on any one or more of the following illustrative and non-exclusive parameters: one or more patient impedance values, the desired quantity of electric charge, shape, phase, timing of phase transition, maximum duration, minimum duration, maximum voltage, minimum voltage, maximum current, minimum current, maximum energy, minimum energy, maximum power, and minimum power.
The present invention in some aspects is a method for delivering a desired quantity of electric charge to a patient. The method includes the steps of determining an impedance of the patient; determining a charge voltage of an energy-storage device based on the impedance and on the desired quantity of electric charge to be delivered to the patient; charging the energy-storage device to the charge voltage; and providing the charged voltage to electrodes in response to a discharge request. This method may also include determining a flow of delivered current into the patient due to discharging the charged voltage. In addition, the steps may be included of determining, based on the flow of delivered current over time, a delivered quantity of electric charge delivered to the patient; continuing discharging the charged voltage into the patient until the delivered quantity of electric charge is equal to the desired quantity of electric charge.
In yet further aspects, the present invention is a defibrillator for delivering a desired quantity of electric charge from an energy storage device to a patient. The defibrillator includes a charge-delivery processor that determines a charge voltage of the energy-storage device based on at least one patient impedance and on delivering the desired quantity of electric charge to be delivered to the patient. The defibrillator also has an applied waveform generator that charges the energy-storage device to the charge voltage determined by the charge-delivery processor. The defibrillator may further include at least one sensor for determining a patient impedance. The applied waveform generator may also discharge the charged voltage into a patient in response to a discharge command. Also, the defibrillator may include a feedback processor that determines, during the discharge of the charged voltage into the patient, an instantaneous quantity of current delivered to the patient due to discharging the charged voltage. In this aspect of the invention, the charge-delivery processor further determines, based on the flow of delivered current over time, a delivered quantity of electric charge delivered to the patient, and determines when the delivered quantity of electric charge is substantially equal to the desired quantity of electric charge. Also in this aspect of the invention, the applied waveform terminates the discharge of the charged voltage into the patient responsive to the charge-delivery processor determining that the delivered quantity of electric charge is substantially equal to the desired quantity of electric charge.
The charge-delivery processor may also determine one or more intended waveform parameters selected from the following illustrative and non-limiting waveform parameters: form, phase, timing of phase transition, maximum duration, minimum duration, maximum voltage, minimum voltage, maximum current, minimum current, maximum energy, minimum energy, maximum power, and minimum power. Also, the applied waveform generator determines the discharge of the charged voltage into the patient responsive to the one or more intended waveform parameters.
In yet another aspect of the invention, a defibrillator is disclosed for delivering a desired quantity of electric charge to a patient. The defibrillator includes a charge-delivery processor that determines one or more intended waveform parameters based at least in part on the desired quantity of electric charge. The defibrillator may also have an applied waveform generator that generates an applied defibrillation waveform based on one or more of the intended waveform parameters.
In a still further aspect of the invention, a defibrillator for delivering a desired quantity of electric charge to a patient is also disclosed. The defibrillator includes a charge-determined waveform that provides a flow of current over time through an electrical coupling to the patient and stops the flow of current when the flow of current over time indicates that the desired quantity of electric charge.
The above aspects and implementations of the invention are not necessarily inclusive or exclusive of each other and may be combined in any manner that is non-conflicting and otherwise possible, whether they be presented in association with a same, or a different, aspect or implementation of the invention. The description of one aspect is not intended to be limiting with respect to other aspects. Also, any one or more function, step, operation, or technique described elsewhere in this specification may, in alternative aspects, be combined with any one or more function, step, operation, or technique described in the summary. Thus, the above aspects are illustrative rather than limiting.